The mixture of methane and nitrogen gases is found in a variety of situations and the mixture must be separated before using the individual component gases. For example, much of the natural gas resources are not readily usable due to high nitrogen (above 10% by volume) content as the commercially usable natural gas must have at least 90% methane gas. Furthermore, the nitrogen content of a reservoir increases with time the reservoir is in use. Methane-nitrogen mixture is also found in fire damp where 27-50% of methane is found. Separation of these gases is difficult due to closeness in their physical properties. At present, commercially this is achieved by energy intensive cryogenic techniques. The application of adsorption based separation of gases by pressure swing adsorption process is being increasingly used now. For example, separation of nitrogen and oxygen from air is in wide prevalence all over the world. Adsorption based processes can compete with highly energy intensive cryogenic separation of methane/nitrogen mixture if a suitable adsorbent which is selective towards one of the components and having adsorption capacity is commercially available.
Characteristics which are highly desirable, if not absolutely essential, for an adsorbent to be suitable for selective adsorption process include adsorption capacity of the adsorbent and adsorption selectivity for a particular component.
Adsorption capacity of the adsorbent is defined as the amount in terms of volume or weight of the desired component adsorbed per unit volume or weight of the adsorbent. The higher the adsorbent's capacity for the desired adsorbing component the better the adsorbent is as the increased adsorption capacity of a particular adsorbent helps to reduce the amount of adsorbent required to separate a specific amount of a component from a mixture of particular concentration. Such a reduction in adsorbent quantity in a specific adsorption process brings down the cost of a separation process.
Adsorption selectivity () of component A over B is defined as EQU .alpha.A/B=X.sub.A Y.sub.B /Y.sub.A X.sub.B
where X is the adsorbed concentration and Y is gas-phase concentration. The expression gas-phase concentration means the amount of unadsorbed component remaining in the gas-phase. The adsorption selectivity of a component depends on
steric factors such as difference in the shape and size of the adsorbate molecules; PA1 equilibrium effect, i.e., when the adsorption isotherms of the components of the gas mixture differ appreciably; PA1 kinetic effect, when the components have substantially different adsorption rates.
It is generally observed that for a process to be commercially economical, the minimum acceptable adsorption selectivity for the desired component is about 3. Where the adsorption selectivity is less than 2, the separation process is not likely to be effective.
In the prior art, methane-selective adsorbent prepared by impregnating molybdenum oxide on activated carbon has been reported. Kinetic separation of methane/nitrogen mixture has also been examined using a naturally occurring zeolite clinoptilolite. The authors have earlier developed faujasite type zeolite based adsorbent for methane-nitrogen separation as disclosed in Indian Patent No. 437/Bom/95, dated Oct. 13, 1995.
The present invention deals with the development of methane selective adsorbents with a new chemical composition based on a different zeolite structure having enhanced adsorption capacity and high adsorption selectivity.
Zeolites which are microporous crystalline aluminosilicates are finding increased application as adsorbents for separating mixtures of closely related compounds. Zeolites have a three dimensional network of basic structural units consisting of SiO.sub.4 and AlO.sub.4 tetrahedra linked to each other by sharing of apical oxygen atoms. Silicon and aluminum atoms lie at the center of the tetrahedra. The resulting aluminosilicate structure which is generally highly porous possesses three dimensional pores the access to which is through molecular sized windows. In a hydrated form, the preferred zeolites are generally represented by the following Formula [I] EQU M.sub.2/n O:Al.sub.n2 O.sub.3 :xSiO.sub.2 :wH.sub.2 O (I)
where "M" is a cation which balances the electrovalence of the tetrahedra and is generally referred to as extra framework exchangeable cation, n represents the valency of the cation, x and w represent the moles of SiO.sub.2 and water respectively. The cations may be any one of the number of cations which will hereinafter be described in detail.
The attributes which made them attractive for separation include, an unusually high thermal and hydrothermal stability, uniform pore structure, easy pore aperture modification and substantial adsorption capacity even at low adsorbate pressures. Furthermore, zeolites can be produced synthetically under relatively moderate hydrothermal conditions.
Pentasil type zeolites as described and defined in U.S. Pat. No. 3,574,539 are the preferred adsorbents for adsorption separation of the gaseous mixture described in this invention. Zeolite of type mordentie in hydrated or partially hydrated form can be described in terms of the following metal oxide of Formula II EQU (0.9.+-.0.2)M.sub.2n O:Al.sub.2 O.sub.3 :(5 to 25)SiO.sub.2 :wH.sub.2 O(II)
where "M" represents at least one cation having valence n, w represents the number of moles of water the value of which depends on the degree of hydration of the zeolite. Normally, the zeolite when synthesized has sodium as exchangeable cation.
Zeolites as such have very little cohesion and it is, therefore, necessary to use appropriate binders to produce the adsorbent in the form of particles such as extrudates, aggregates, spheres or granules to suit commercial applications. Zeolitic content of the adsorbent particles vary from 60 wt % to 98 wt % depending on the type of binder used. Clays such as bentonite, kaolin, or attapulgite are normally used inorganic binders for agglomeration of zeolite powders.